Supplier Threat Protection prioritization focuses on vendor identities whose messaging patterns indicate elevated risk---such as unusual sending behavior, higher malicious/suspicious message counts, abnormal spike patterns, or stronger impersonation/compromise indicators relative to other suppliers. Based on the exhibit's Notable Senders metrics, bob@aerowestglobalservices.com (C) shows the highest-risk activity and should be investigated first. In Proofpoint IR workflow, supplier-related threats are high impact because they exploit trust relationships and can bypass user suspicion (invoice/payment workflows, shared documents, ongoing threads). The investigation typically validates whether this is: (1) a compromised supplier mailbox, (2) supplier-domain impersonation (lookalike domain), or (3) a legitimate supplier system misconfigured and sending risky content. Analysts pivot into message samples, authentication alignment (SPF/DKIM/DMARC), sending infrastructure changes, and recipient targeting patterns (finance/AP, executives). If malicious, containment includes blocking the supplier sender/domain (or precise subdomains), pulling delivered copies via TRAP, alerting impacted users, and initiating vendor contact to remediate the supplier's account security.